overnight margin change time

I trade mostly ES and FESX on IB but recently i have been trading ZN and bund contracts.

today i had a ZN trade and a bund trade on into close and at around 3:30 i got the overnight margin warning. does anyone know which contract this was for? i know the ES and YM switch to overnight at 4pm, but does any1 know what time it is for ZN and bunds?